1. Field
The system and method of the present invention pertains to the field of heaters for fluids; more particularly, the inline heating of a fluids in a confined space without introducing contaminates to the fluid being heated.
2. Background
Heated ultrapure fluids are used for a variety of reasons. For example, hot fluids are required during several processing steps in the manufacture of an integrated circuit. It is typically impractical to first heat the fluids and then purify it and, because of the miniaturized scale of microcircuits and the critical manufacturing tolerances required in their production, virtually any impurity in the etching or rinsing fluid can result in defective parts and, consequently, wasted resources. Accordingly, it is preferable to start with a pure fluid and then heat it to the desired temperature.
Traditional heat exchange systems are unable to meet the demands of today's integrated circuit manufacturing process. For example, in a coil heat exchanger, a long, small diameter tube is placed concentrically within a larger tube, the combined tubes being bent or wound in a helix. A fluid of one temperature passes through the inner tube, and a second fluid of another temperature passes through the outer tube. The heat exchanger can be configured so that the liquid in the inner tube heats or cools the liquid in the outer tube or vice versa. This type of heat exchanger is generally capable of handling high pressures and wide temperature differences. Although these exchangers tend to be quite inexpensive, they tend to be quite large, they provide rather poor thermal performance because of the small heat transfer area, and they are antagonistic to ultrapure liquids.
Another traditional heat exchanger, the shell-and-tube type heat exchanger, consist of a bundle of parallel tubes that provide the heat transfer surface separating two fluid streams. The tube-side fluid passes axially through the inside of the tubes while the shell-side fluid passes over the outside of the tubes. Baffles external and perpendicular to the tubes direct the flow across the tubes and provide tube support. The shell-and-tube exchanger is efficacious in certain circumstances but has severe limitations in connection with integrated circuit processing, including the large size of the exchanger, thermal inefficiency and general intolerance for ultrapure liquids.
Heater manufacturers have sought to design devices acceptable for integrated circuit manufacturing which are thermally efficient, responsive to fluid flow changes, and capable of long life. For example, in order to maintain the purity required in integrated circuit processing filtering processes are employed to remove contaminants and de-ionize the fluid. Heat exchange systems are also generally designed to prevent contact between the contaminant-free fluid and any substance that would tend to corrode in the presence of the fluid, causing impurities to be reintroduced. Although most plastic materials tend to be good thermal insulators and therefore seemingly inappropriate for some uses in heating systems, most modem heaters for use in microchip manufacturing systems must employ plastics barriers to prevent the contaminant-free fluid from contacting the metallic heating element, lead wires and the like.
The prior art teaches a number of techniques for heating ultra-pure liquids. For example, in U.S. Pat. No. 4,461,347, issued Jul. 24, 1984, Layton et al., teaches immersing a heat source within a stream of the fluid to be heated. In this process, the heating element is contained within a non-reactive material to prevent contamination of the fluid. Heat is transferred to the fluid by conduction. As the heat from the heat source increases, the likelihood of contamination increases. Layton also teaches that the non-reactive sheath is preferably a plastic such as polytetraflouroethylene or polypropylene, both of which are thermally insulative, thereby reducing the efficiency of the transfer of heat to the fluid.
In U.S. Pat. No. 4,797,535, issued Jan. 10, 1989, Martin teaches heating a fluid by immersing a tungsten-halogen bulb in the fluid within a vessel, such as a pipe. As the fluid passes the bulb, heat transfers to the fluid. Martin does not appear to contemplate ultra-pure fluids, and no precautions are taken or taught for maintaining the purity of the fluid.
In U.S. Pat. No. 5,054,107, issued Oct. 1, 1991 Batchelder teaches a system for heating ultra-pure fluids. In particular, a quartz spiral or double walled tube is configured to surround several high intensity lamps. The fluid to be heated flows through the quartz tube. The lamps are not immersed in the fluid but radiate energy (infrared) outward through the tube and the liquid. The construction is wrapped in aluminum foil to reflect radiation that passes beyond the tube back through the fluid.
It is well recognized that the operative life of lamps of this type is greatly diminished as a result of high temperature operating conditions. Batchelder appears to recognize this and discloses a fixture for removing heat from the ends of the bulbs. Nevertheless, Batchelder teaches that up to twelve lamps can be mounted within the center of the quartz tube. These lamps will necessarily heat one another, thereby reducing the effective lifetime for the system, requiring more frequent routine maintenance for lamp replacement.
In U.S. Pat. No. 5,790,752, Anglin, et. al. teach a system for heating ultrapure liquids utilizing one or more elongated lamps that generate infrared radiation as the heating elements. In particular, the infrared lamps surround a vessel made of quartz through which liquid that is to be heated is passed. A quartz vessel, such as tubing, can be expensive and difficult to form into the desired configuration. In addition, the mass of the quartz present also absorbs some percentage of the infrared energy and keeps that amount of energy from being absorbed by the liquid being heated.
Accordingly, there exists a need for non-contaminating fluid heating systems which can efficiently and economically heat and maintain the fluid passing therethrough at a desired temperature. Further, a fluid heater is needed which is durable and capable of long, sustained use in harsh environments. Moreover, a fluid heater and control system is needed for preventing damage to the heater components and for ensuring that the fluid will be heated only to temperatures within acceptable limits. The present invention fulfills these needs and provides other related advantages.